[comctl32] Fix SEGV in file dialogs (bug 3366)

[Troy Rollo]

The following change seems to prevent the SEGV that frequently occurs when you 
double-click on a folder in the file open/save dialogs. This fix uses 
IsWindow() to test if the ListView still exists. I have handled all 
notifications other than those where it seems entirely implausible that the 
callback could destroy the window.

ChangeLog:
	Detect when the notification callback has destroyed the ListView to avoid
	attempts to access data that is no longer valid.
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